Generally, coordinated beam-forming (CBF), also referred to as Coordinated Scheduling/Beamforming, is an effective yet cost efficient way to improve cell-edge throughput. Its main objective is to eliminate the flashlight effect in the interference caused by closed-loop multiple input, multiple output (CL-MIMO) operation, so as to restore the effectiveness in link adaptation. Another objective, although to less extent, is to reduce inter-cell interference (ICI), so as to improve the average throughput promised by user equipment (UE) measured signal to interference plus noise ratio (SINR).
Since in frequency division duplex (FDD) communications systems, the transmitter does not know the channel in the downlink (DL), limited channel state information (CSI) needs to be sent back by the UE to the transmitter. However, it has been agreed in the Third Generation Partnership Project (3GPP) Technical Specifications Group Radio Access Network Working Group One (TSG-RAN WG1) #60 meeting that no additional features are to be specified in Release-10 of 3GPP Long Term Evolution (LTE) technical standards to support DL coordinated multipoint (CoMP) transmission, which makes DL CoMP related UE feedback unavailable. To make CBF work, channel reciprocity between the uplink (UL) and the DL may be used.